1. Field of the Invention
This invention relates to the recovery of starch from the cellular tissue of root crops such as manioc, potatoes, yams, turnips, beets, carrots, and the like and more particularly, to the recovery of starch from such root crops in the form of an aqueous slurry.
2. Description of the Prior Art
With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versatility.
The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquifaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy and raw materials as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy and raw material requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure and waste incurred in carrying out any of the aforesaid discrete operations involved in the manufacture of ethanol from vegetative sources.
The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources. The most common such operation employs cane sugar in a fermentation-distillation operation which conveniently utilizes the bagasse by-product as a fuel source. Cassava or manioc (Manihot utilissima Pohl) as a source of starch has also been considered for conversion into alcohol (see "Brazil's National Alcohol Programme", Jackson, ed. Process Biochemistry, June 1976, pages 29-30; "Ethyl Alcohol from Cassava", Teixeira et al., Industrial and Engineering Chemistry pp. 1781-1783 (1950); and United Kingdom Patent Specification No. 1,277,002). Manioc root is an especially attractive source of starch since it produces a very large yield per acre of tubers rich in total carbohydrates and grows well upon soils where no other crop does well with very little tending. However, since manioc lacks the equivalent of sugar cane's bagasse, the fuel for alcohol conversion must come from an external source. Thus, to make manioc root and other amylaceous roots economically attractive sources of ethanol, it is essential to utilize as much of the carbohydrate content of the root as possible and to achieve rapid and high levels of conversion of recovered starch to fermentable saccharide and of the fermentable saccharide to ethanol with high levels of thermal efficiency, minimum raw material waste and low plant construction and operating costs. Manioc root typically assays as follows:
______________________________________ Component Weight Percent ______________________________________ Starch 30 Cellulose 3 Protein 3 Soluble carbohydrate 3 Ash 1 Water 60 ______________________________________
In addition to soluble carbohydrates which include gums, pectins and sugars, manioc root and other amylaceous roots contain significant quantities of soluble proteins. Under the process conditions of known starch recovery procedures (viz., U.S. Pat. Nos. 1,016,762; 1,156,801, 2,135,104; 2,380,874; 2,798,011; 2,974,068; 3,072,501; 3,079,283; 3,433,668; and 3,948,677) these soluble components present in solution in the process water (so-called "fruit water") are discarded or otherwise separated from the starch. While this loss of soluble carbohydrates and proteins is not considered a problem in the starch and sugar industries where the purity and quality of the end product is of paramount importance, it represents a substantial disadvantage to the full and efficient utilization of root starch as a raw material for the production of inexpensive industrial ethanol. The loss of soluble carbohydrate which represents nearly ten weight percent of the total quantity of carbohydrate present in manioc root accounts for a substantial waste of useful raw material. Similarly, the loss of soluble protein which would otherwise be available to satisfy the nutritive requirements of yeast employed in the conversion of fermentable sugar obtained by hydrolysis of the starch further militates against the use of current procedures for the recovery of starch from roots.
Accordingly, there has heretofore existed a need for a process for recovering starch from amylaceous roots which does not result in the removal of substantial quantities of water soluble carbohydrate and/or protein therefrom.